1. Field of the Invention
The present invention relates to a method and apparatus for recording information on an optical recording medium, and in particular, to a method for irradiation the optical recording medium with a laser beam using pulse control during recording.
2. Description of the Related Art
Various standards of optical recording media such as CD-R/RWs and DVD-R/RWs, which allow the user to record information, have been widely used. Meanwhile, the demand for large storage capacity in these types of optical recording media is increasing year by year. In order to meet this demand, new standards such as Blu-ray discs (BDs) have been proposed. In the Blu-ray Disc standard, an optical disc apparatus uses a data recording/reproduction laser beam with a narrow beam spot diameter. Specifically, the apparatus uses a laser beam with a shorter wavelength λ and a higher numerical aperture (NA) objective lens for condensing the laser beam. As a result, an information recording layer of the Blu-ray disc can store as much data as 25 GB or greater.
In general, rewritable optical recording media, which allow rewriting data, use a recording film made of a phase change material. Specifically, heating the recording film by irradiation with a laser beam and appropriately controlling the cooling rate thereafter can selectively form an amorphous region and a crystal region in the film, and the difference in reflectivity between these regions achieves recording information. When doing so, the laser beam is required to be set under various conditions such as a write power level (Pw) with a high energy, an erase power level (Pe) with a middle energy, and a bias power level (Pb) with a low energy. Recording information on the optical recording medium is performed by pulse irradiation of the laser beam with any selected one of the three power levels.
In order to form a mark on an optical recording medium, a write pulse at the write power level and a bias pulse at the bias power level are alternately irradiated. The recording film is irradiated with the write pulse, so that the irradiated region is heated to its melting point or more. When the same region is then irradiated with the bias pulse, the area is rapidly cooled and thus is turned to an amorphous recording mark. Therefore, if the set of the write pulse and the bias pulse is continuously applied, it is possible to form a long recording mark.
Irradiating the optical recording medium with a laser beam with the erase power level erases a mark having been recorded on the optical recording medium. The recording film is irradiated with the laser beam with the erase level, so that the irradiated region is heated to its crystallization temperature or more. The whole irradiated region, including the amorphous region (mark), is then crystallized by natural cooling, so that the recording mark is erased.
Accordingly, recording information is performed in such a manner that a region where a mark is to be formed is continuously irradiated alternately with the write pulse and the bias pulse according to the length of the mark, and a region where a space is to be formed is irradiated with a laser beam with the erase power level according to the length of the space. Such a power modulation method is called a write strategy.
Increasing the recording density as well as the write speed causes an edge shift on the recording mark. For example, when a long recording mark such as 4 T or 6 T is formed by a plurality of write pulses, too high write speed makes it hard to ensure sufficient cooling time, which corresponds to the width of the bias pulse between the write pulses. This poor cooling causes a part of the mark to recrystallize, and thus may degrade the recording quality. Therefore, in order to increase the recording accuracy, accurate control of the laser pulse is required, and therefore, there are various approaches.
For example, Japanese Patent Application Laid-Open No. 2005-71516 discloses that, when writing each mark, the width of a bias pulse which is inserted into the end of the mark is varied depending on the length of the mark so as to reduce the jitter of a production signal. Moreover, when writing each mark, a bias pulse is inserted additionally into the top of the mark so as to delay the rising of the first write pulse. Similarly, Japanese Patent Application Laid-Open Nos. 2005-63586 and 2002-288830 disclose that at the beginning of writing a mark, a pulse with a power level lower than that of an erase pulse is inserted so as to prevent the top area of the mark from recrystallizing.
Japanese Patent Application Laid-Open No. 2001-273638 discloses that, when forming a mark with a 4 T length or more, a bias pulse is inserted so as to prevent the mark from recrystallizing. The publication of Japanese Patent No. 2707774 discloses that, when forming a long mark by, for example, three write pulses or more, a bias pulse is inserted either before a top write pulse or after an end write pulse so as to sharpen the leading or trailing edge of the mark.
However, a further increase in storage capacity increases the recording density of the information recording layer, thereby degrading the quality of the reproduction signal. This makes it hard to identify a bit using zero-crossing detection. At the same time, this makes it hard to determine the signal quality using the jitter. Therefore, a Partial Response, Maximum Likelihood (PRML) detection scheme may be used for signal reproduction. However, if the PRML detection scheme is adopted, then errors associated with marks corresponding to 2 T or 3 T are likely to increase. In other words, errors are likely to occur when the recording pattern is made of a series of relatively short marks. All the patent documents mentioned above are to perform a precise pulse control of all marks or relatively long marks. Therefore, the application of such a control without any modification is unable to fully perform high density recording.
Moreover, when high-speed recording is to be conducted on a large capacity (high density) medium exceeding 30 GB, it is necessary that the recording film be made of a material whose crystallization speed is extremely fast. However, if low-speed recording is conducted on such an optical recording medium, the recording accuracy will significantly deteriorate.